Magneto-optical disks are widely used as external memories for computers etc. A mechanical clamping system and a magnetic clamping system are adopted for mounting magneto-optical disks on recording and reproducing apparatus. Unlike the mechanical clamping system, the magnetic clamping system needs no device for fixing a magneto-optical disk. As shown in FIG. 1, the magnetic clamping system involves supporting a substrate 1 on the support surface 2a of a spindle 2. The substrate 1 is fitted with a hub 3. The spindle 2 includes a magnet 4 embedded in it. This system also involves making the magnet 4 attract the hub 3 to urge an inner circumferential portion of the reverse side of the substrate 1 against the support surface of the spindle 2. This system can make the apparatus small in comparison with the mechanical clamping system, but has the problem of retardation that the difference between the coefficients of thermal expansion of the plastic substrate and the hub develops a deforming stress in the substrate when the ambient temperature changes. This retardation disturbs the states of polarization of the recording light and reproducing light for the magneto-optical disk, and may consequently degrade the recording and reproducing characteristics.
As a means to solve this problem, a structure of an optical disk as shown in FIG. 2 is proposed in Japanese Patent Application Laid-Open No. 62-46446, for example. This optical disk includes a magnetic clamping hub 3, which is not fixed to a substrate 1 but can move relative to it. As shown in FIG. 2, a spindle 2 is fitted with a magnet 4, which attracts the clamping hub 3 and simultaneously urges it against the substrate 1 so that the disk can rotate with the spindle 2. Since the clamping hub 3 and the substrate 1 are not fixed together, there occurs no retardation resulting from the difference between their coefficients of thermal expansion.
However, researches of the inventors revealed that, when a medium on the magnetic clamping system is rotated in a recording and reproducing apparatus at a speed higher than 720 rpm for a higher transfer rate, the apparatus vibrates, frequently causing tracking errors and/or data write/read errors. The researches also revealed that this phenomenon is more remarkable when the medium rotates at higher speed. This is conceived to be because the magneto-optical disk rotates irregularly due to the slip caused since the torque of the disk is greater than the frictional force acting between the substrate and the magnetic clamping hub. Furthermore, the above-mentioned errors result from the camming and/or the disordered flow of air (turbulence) caused by the irregular rotation.
If the substrate is thinner for higher record density, it is less rigid. This makes the torque of the spindle apt to cause camming and deformation of the disk. The camming and the deformation may make the disk rotation irregular and cause turbulence.
If the magneto-optical disk rotates at higher speed in a cartridge case, the disk rotation causes more turbulence in the cartridge. The turbulence is conceived to affect the rotational stability of the disk. The turbulence is also conceived to increase the rotational instability and camming of the disk.
When the disk rotates at higher speed, the spindle motor generates a larger amount of heat than conventionally. The disk is conceived to be affected by a great amount of heat transfer to its substrate. While the disk is rotating at high speed, it is charged by the friction between the disk surfaces and air. This is conceived to make a larger amount of dust stick to the disk. In particular, because an optical disk including a thin substrate is apt to be charged, a remarkable amount of dust sticks to the disk.
Then, the inventors tried to prevent the slip between a substrate and a magnetic clamping hub by making the magnetic attraction of a spindle greater than conventionally by replacing the magnet of the spindle with one greater in magnetic force, but the signal quality deteriorated. This is conceived to be because the too strong magnetic field from the spindle magnet affects the magneto-optical signals.
Information can be recorded on and reproduced from a magneto-optical disk or another optical record medium with light radiated onto the medium. The angle between the optical axis of the incident light and the medium (recording layer) influences the recording and reproducing performance very greatly. Therefore, when information is recorded and reproduced, it is most preferable that the light be incident at right angles with the recording surface of the optical record medium. In many actual cases, however, the angle between the recording surface of an optical record medium and the incident light is not right because of the substrate warpage caused when the medium is manufactured, setting errors of the optical system of a driving apparatus, and the disk distortion due to changes in environmental temperature and humidity. In particular, a plastic substrate for an optical record medium which is 0.8 mm or thinner is less rigid. It is therefore difficult to prevent the substrate from being warped and/or distorted.
The angle between the medium surface and the incident light greatly affects the coma aberration, which increases in proportion to the third power of the angle of inclination from the perpendicular direction. In other words, if the medium surface inclines with respect to the optical axis of the incident light, the focal spot becomes substantially large in diameter. This may prevent a desired record mark from being recorded. If coma aberration is produced when information is reproduced, it is difficult to reproduce a recorded mark with sufficient resolution.
Conventionally, the problem which originates from the tilt of a desired position on a surface of a medium with respect to the optical axis of incident light has been met with the precision assembly technique for the optical system of a driving apparatus and the technique for making flat optical record media. This problem of the tilt may be solved by a known method called a tilt servo. The method includes detecting the tilt of an optical record medium through the optical system of a driving apparatus, and moving the optical system itself, to control the optical axis of the incident light so that the axis is perpendicular to the medium surface. This method, however, requires a drive mechanism for moving the optical system accurately. This makes the apparatus structure complex and costly. This method also requires a certain amount of time for driving the optical system to make the optical axis of the incident light perpendicular to the medium surface. This makes it difficult to manufacture a quickly responsive recording and reproducing apparatus.
In consideration of the foregoing problems, a first object of the present invention is to provide an optical disk for which it is possible to suppress camming and irregular rotation by preventing the substrate and hub of the disk from slipping on each other when the disk is rotated at high speed.
A second object of the present invention is to provide a substrate for a record disk having a recording surface that can be perpendicular to the optical axis of the light incident on the surface when information is recorded on and reproduced from the disk.
A third object of the present invention is to provide a driving apparatus that can keep a record disk horizontal by correcting the tilt of the disk while the disk is rotated.